1. Field of the Invention
This invention relates to a method of making a sheet electrode for an electric double layer capacitor by rolling a sheet intermediate using rollers and a roller rolling machine suitable for use in the method.
2. Description of the Related Art
Electric double layer capacitors have a large capacity and an excellent charge and discharge cycle property. Therefore, their application to various appliances has been examined. The application includes backup power supplies for various electronic devices and batteries for means of transportation such as automobiles.
The above-mentioned electric double layer capacitors include a cylindrical type. In the cylindrical electric double layer capacitor, a pair of positive and negative electrodes each comprise a sheet electrode (polarizable electrode) held on a collecting electrode such as aluminum foil. The paired electrodes are wound into a coil with a separator interposed therebetween. The wound paired electrodes are accommodated in a cylindrical case while being impregnated with electrolyte.
The aforesaid sheet electrode is obtained by kneading carbon powder such as activated carbon, conductive assistant such as carbon black, a binder such as fluorocarbon resin, a binding assistant comprising a liquid lubricant such as alcohol, thereby obtaining a kneaded material. The kneaded material is formed into the aforesaid sheet electrode. JP-A-2000-150321 discloses rolling the above-mentioned kneaded material with use of rollers. Furthermore, JP-A-2-235320 discloses another rolling method with use of rollers. The kneaded material is rolled by rollers to be formed into a preparatory intermediate having a thickness of 1 mm. The preparatory intermediate is dried so that the liquid lubricant is eliminated. Thereafter, the preparatory intermediate is re-rolled by the rollers to be formed into a sheet electrode having a thickness of 0.25 mm.
A long continuous sheet electrode is required in order that a cylindrical electric double layer capacitor with a large capacity as described above may be made. Furthermore, the sheet electrode is desired to have a sufficiently small thickness and a sufficient accuracy. However, the material comprising activated carbon as a main component has a low strength and accordingly, ends of the sheet electrode are easy to be broken or cracked. Thus, it is difficult to make a sufficiently thin and long sheet electrode by rolling. The above-cited JP-A-2-235320 further discloses that an end of the preparatory intermediate is placed on an end of another intermediate so that the rolled sheet electrode is rendered longer. However, this manner complicates the rolling.